1. Field of the Invention
The invention relates to an antenna device for a radio communication device, comprising a conductive first plate, having a first surface, being arranged to be connected to signal ground of the radio communication device; a conductive second plate, having first and second opposing edges partially limiting a second surface of the second plate, being arranged spaced apart from the first plate, the first and second surfaces facing each other; and a conductive grounding means interconnecting the first and second plates at least along a portion of the first edge of the second plate.
Especially it relates to an antenna device including a housing cover to be attached to a glass pane, e.g. a top portion of a wind screen or a back window of an automobile.
2. Related Art
WO-A1-97/44856 and EP-A1-0856907 disclose antenna devices of the type described in the first paragraph above. Those antenna devices are aperture-coupled planar antenna devices for mobile radio communication devices, e.g., hand-portable telephones. Antenna devices therein are fed by a feed conductor exciting an aperture, creating an electromagnetic field across the aperture. They can be made small and compact so as to fit in a mobile radio communication device, such as a hand-portable telephone. It is well known that the size of an antenna is critical for its performance (see Johnsson, Antenna Engineering Handbook, McGrawHill 1993, chapter 6). This can be expressed as a limitation of the product of the relative bandwidth (.DELTA.f/f) and the efficiency (.eta.), which always is smaller than a constant multiplied by the efficient volume (V) of the antenna (as expressed in cubic wavelengths): EQU (.DELTA.f/f).eta.&lt;constant(V/.lambda..sup.3)
It is therefore important for small antennas that the available volume is effectively used.
Further it is disclosed through EP-A2-0 821 429, JP-A-10016646, EP-A2-0 841 715, and DE-A1-196 16 974 different solutions of how to accommodate an antenna device close to a glass pane or similar in an automobile. These antenna devices do not appear to be very efficient or they are of a complicated construction, which makes them complicated to manufacture. Further, they only provide single band operation.